A hybrid vehicle may convert a vehicle's kinetic energy into electrical energy via regenerative braking. The electrical energy may be stored in an electrical energy storage device until it is subsequently used to propel the vehicle. For example, electrical energy stored during vehicle braking may be subsequently used to launch the vehicle or to increase vehicle performance during high vehicle load conditions. However, during some conditions, regenerative braking may be temporarily deactivated in favor of friction braking. For example, regenerative braking may be deactivated and friction braking may be activated during conditions when wheel slip is detected while the vehicle is braking. Deactivating regenerative braking may simplify vehicle braking; however, a portion of the vehicle's kinetic energy may be lost to heat instead of being preferentially converted into electrical energy that may be stored for subsequent use. As a result, the vehicle's driving range may be reduced.
The inventors herein have recognized the above-mentioned issues and have developed a vehicle operating method, comprising: reducing speed of a first wheel via regenerative braking and friction braking; and reducing the friction braking of the first wheel in response to slip of the first wheel without reducing the regenerative braking of the first wheel.
By prioritizing regenerative braking over friction braking during some conditions, it may be possible to control vehicle wheel slip while converting a vehicle's kinetic energy into electrical power. Further, friction brakes may augment regenerative braking during conditions of wheel slip when regenerative braking capacity may be limited. The regenerative braking may be adjusted responsive to two different wheel torques so that slip for individual wheels may be controlled. The electrical energy generated during regenerative braking may be used at a later time to propel the vehicle and conserve fuel.
The present description may provide several advantages. In particular, the approach provides torque control for each wheel coupled to an axle. The torque may be controlled via friction braking or an electric machine. Further, the approach adjusts friction braking responsive to regenerative braking so that a desired wheel torque may be provided while at least one wheel of a vehicle is slipping. In addition, the approach compensates wheel torque control during conditions where regenerative braking may be limited because unusual vehicle operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.